1. Field
The disclosure relates to a method, system, and article of manufacture for consistent updates across storage subsystems.
2. Background
Information technology systems, including storage systems, may need protection from site disasters or outages. Furthermore, information technology systems may require features for data migration, data backup, or data duplication. Implementations for disaster or outage recovery, data migration, data backup, and data duplication may include mirroring or copying of data in storage systems. Enterprise storage servers (ESS) may include one or more processors coupled to storage devices, including high capacity scalable storage devices, Redundant Array of Independent Disks (RAID), etc. Enterprise storage servers may include copy functions for copying data either locally, i.e., within the same on-site ESS, or remotely, i.e., copying data to a separate remote-site ESS.
In certain enterprise storage servers there may be copy functions that provide a point-in-time copy of the data. Implementations may copy data between a set of local/source volumes and a corresponding set of remote/target volumes in storage subsystems in enterprise storage servers. Copy functions may provide a point-in-time copy for ESS volumes by creating a physical point-in-time copy of the data, with minimal interruption to applications, and make it possible to access both the source and target copies substantially immediately.
A consistency group is a group of storage volumes that need to be kept in a consistent state with each other. To provide a non-limiting example, a first copy command copies volume A1 to B1 and a second copy command copies volume A2 to B2. It is required that volumes B1 and B2 should represent a consistent state of the dataset in volumes A1 and A2 at a certain point in time. In a certain sequence of operations on the volumes the following set of dependent write operations may occur (where the second operation occurs after the first operation):    1. Write to dataset on volume A1 (data updated)    2. Write to dataset on volume A2 (data updated)
When volumes A1 and A2 are copied to volumes B1 and B2 respectively, then the following non-limiting example of a sequence of operations may create an inconsistent state in volumes B1, B2 with respect to volumes A1, A2.    1. Copy volume A1 to volume B1    2. Write to dataset on volume A1 (data updated)    3. Write to dataset on volume A2 (data updated)    4. Copy volume A2 to volume B2
At the conclusion of all the copy operations, i.e., the conclusion of the fourth operation, volume B2 contains the data update of volume A2 whereas volume B1 does not contain the data update of volume A1. The set of volumes B1, B2 are in an inconsistent state with respect to the set of volumes A1, A2. An application that uses the volumes B1, B2 could not recover from a back-up copy stored in the volumes B1, B2. stored in a disk coupled to a computer can be backed up by generating an image of the disk.
Certain systems copy a set of source volumes to a set of target volumes in storage subsystems, while maintaining the data on the set of source volumes and target volumes in storage subsystems in a consistent state with respect to each other. Maintaining source and target volumes in storage subsystems in a consistent state ensures that no out of order dependent writes are copied to any target volumes in storage subsystems in a consistency group.